It is well known that one of the major limitations in cellular and other wireless telephone networks is so-called co-channel interference. In the case of time division multiple access (TDMA) networks, such as GSM (Global System for Mobile Communications)/GPRS (General Packet Radio Service)/EDGE (Enhanced Data Rates For GSM Evolution), co-channel interference is caused mainly by what is termed frequency reuse, i.e. the same frequencies in the allocated spectrum are used in different cells, and so, depending on the reuse factor, a signal received by a cellular phone will contain not only a desired forward channel for a current cell, but also channels originating in other cells.
The prior art for co-channel interference suppression includes: “A Receiver,” having International Publication Number WO 01/93439, with priority date of 31 May 2000, by inventors Ottersten, Kristensson, and Astely; and also “Methods and Apparatus for canceling co-channel interference in a receiving system using spatio-temporal whitening,” having U.S. patent application Pub. No. US 2003/0063596, with a priority date of Sep. 19, 2001, by inventors Huseyin Arslan and Ali S. Khayrallah. The receiver disclosed in WO 01/93439 is based on the well-known fact that if (co-channel) interference is considered to be colored noise and the noise is whitened, signal gain can be achieved. The novelty of WO 01/93439 is asserted to be a filter providing efficient whitening by exploiting the additional degree of freedom that arises from the separation of the real and imaginary components of the received signal, i.e. of the in-phase and quadrature-phase components. The teachings of US 2003/0063596 are similar to WO 01/93439 in respect to suppressing co-channel interference.
In WO 01/93439 the interference is modeled as an IIR (infinite impulse response) process with order K, and (consequently) the whitening operation is performed by a (multidimensional) FIR (finite impulse response) filter with K (or K+1) filter taps. After the whitening operation, the impulse response of the wanted signal is of course modified; in particular, because of the convolution with the whitening filter, the whitening operation of WO 01/93439 suffers from what is here called “the increased channel length problem,” i.e. the impulse response of the wanted signal becomes longer, requiring a more complex equalizer or at least a modified equalizer including some mechanism to take into account the increased channel length (otherwise the receiver incurs a performance loss). The increased channel length problem requires that the equalizer of a receiver be modified if the whitening operation per WO 01/93439 is to be implemented by the receiver.
Additionally the achievable performance gain obtainable using the whitening operation of WO 01/93439 depends on the model parameter K indicating the number of taps of the FIR filter. In general the higher the K, the higher the gain, but if K exceed a certain threshold (which depends on the particular interference being suppressed and so is in principle not a priori known) the problem of finding the FIR filter coefficients becomes ill-conditioned, i.e. the FIR filter can not be found.
What is therefore needed is a more robust, less complex method of suppressing co-channel interference based on noise whitening, one that is easier to integrate into existing receivers, such a GSM (Global System for Mobile Communications)/EDGE (Enhanced Data Rates for GSM Evolution) receivers.